The invention relates to a polyacrylate pressure sensitive adhesive (PSA), to a process for preparing such a PSA, and to the use of such PSAs.
For industrial PSA tape applications it is very common to use polyacrylate PSAs. Polyacrylates possess a variety of advantages over other elastomers. They are highly stable toward UV light, oxygen, and ozone. Synthetic and natural rubber adhesives normally contain double bonds, which make these adhesives unstable to the aforementioned environmental effects. Further advantages of polyacrylates include their transparency and their serviceability within a relatively wide temperature range.
Polyacrylate PSAs are generally prepared in solution by free radical polymerization. The polyacrylates are generally applied to the corresponding backing material from solution using a coating bar, and then dried. In order to increase the cohesion, the polymer is crosslinked. Curing takes place thermally or by UV crosslinking or by EB curing (EB: electron beams). The process described is fairly costly and ecologically objectionable, since as a general rule the solvent is not recycled and the high consumption of organic solvents represents a high environmental burden.
Moreover, it is very difficult to produce PSA tapes with a high adhesive application rate, without bubbles.
One remedy to these disadvantages is the hotmelt process. In this process, the PSA is applied to the backing material from the melt.
However, this new technology has its limitations. Prior to coating, the solvent is removed from the PSA in a drying extruder. The drying process is associated with a relatively high temperature and shearing effect, so that high molecular mass polyacrylate PSAs in particular are severely damaged. The acrylic PSA gels, or the low molecular mass fraction is greatly enriched as a result of molecular weight breakdown. Both effects are undesirable, since they are disadvantageous for the application. Either the adhesive can no longer be applied or there are changes in its technical adhesive properties, since, for example, when a shearing force acts on the adhesive the low molecular mass fractions act as lubricants and so lead to premature failure of the adhesive.
One solution to mitigating these disadvantages is offered by polyacrylate adhesives with a low average molecular weight and narrow molecular weight distribution. In this case the fraction of low molecular mass and high molecular mass molecules in the polymer is greatly reduced by the polymerization process. The absence of the high molecular mass fractions reduces the flow viscosity, and the adhesive shows less of a tendency to gel. As a result of the reduction in the low molecular mass fraction, the number of oligomers which reduce the shear strength of the PSA is lessened.
A further disadvantage of relatively low molecular mass acrylic PSAs is the relatively low crosslinking propensity. Short polymer chains are generally crosslinked less efficiently, since there is a lesser probability that a polymer radical will meet a second polymer chain. In order to increase the crosslinking efficiency, therefore, promoters are needed.
A variety of polymerization methods are suitable for preparing low molecular mass PSAs. The state of the art is to use regulators, such as alcohols or thiols, for example (Makromolekxc3xcle, Hans-Georg Elias, 5th Edition, 1990, Hxc3xcthig and Wepf Verlag Basel). These regulators reduce the molecular weight but broaden the molecular weight distribution.
Another controlled polymerization method used is that of atom transfer radical polymerization (ATRP), in which initiators used preferably include monofunctional or difunctional secondary or tertiary halides and, for abstracting the halide(s), complexes of Cu, Ni, Fe, Pd, Pt, Ru, Os, Rh, Co, Ir, Cu, Ag or Au [EP 0 824 111; EP 0 826 698; EP 0 824 110; EP 0 841 346; EP 0 850 957]. The various possibilities of ATRP are further described in U.S. Pat. Nos. 5,945,491, 5,854,364 and 5,789,487. Generally, metal catalysts are used, which have the side effect of adversely influencing the aging of the PSAs (gelling, transesterification). Moreover, the majority of metal catalysts are toxic, discolor the adhesive, and can be removed from the polymer only by means of complicated precipitations.
U.S. Pat. No. 4,581,429 discloses a controlled radical polymerization process. As its initiator the process employs a compound of the formula Rxe2x80x2Rxe2x80x3Nxe2x80x94Oxe2x80x94X, in which X denotes a free radical species which is able to polymerize unsaturated monomers. In general, however, the reactions have low conversion rates. A particular problem is the polymerization of acrylates, which takes place only with very low yields and molecular weights.
WO 98/13392 describes open-chain alkoxyamine compounds which have a symmetrical substitution pattern. EP 0 735 052 A1 discloses a process for preparing thermoplastic polymers having narrow polydispersities.
WO 96/24620 describes a polymerization process in which very specific radical compounds, such as phosphorus-containing nitroxides, for example, are described.
WO 98/30601 discloses specific nitroxyls, based on imidazolidine.
WO 98/4408 discloses specific nitroxyls based on morpholines, piperazinones, and piperazinediones.
DE 199 49 352 A1 discloses heterocyclic alkoxyamines as regulators in controlled radical polymerizations.
Corresponding further developments of the alkoxyamines or of the corresponding free nitroxides improved the efficiency for the preparation of polyacrylates. [Hawker, C. J., Paper, National Meeting of the American Chemical Society in San Francisco, Spring 1997; Husemann, M., IUPAC World Polymer Meeting 1998, Gold Coast, Australia, Paper on xe2x80x9cNovel Approaches to Polymeric Brushes using xe2x80x98Livingxe2x80x99 Free Radical Polymerizationsxe2x80x9d (July 1998).]
In the abovementioned patents and papers attempts were made to improve the control of radical polymerization reactions. There nevertheless exists a need for a nitroxide-controlled polymerization process which is highly reactive and can be used to realize high conversions in combination with high molecular weight and low polydispersity. These requirements have been met in DE 100 36 801.8.
Nevertheless, this type of compound does not promote efficient crosslinking. Instead, nitroxides generally act as carbon radical scavengers and therefore possess an inhibitory effect as far as crosslinking is concerned.
BASF AG offers UV AC-Resins(trademark) containing copolymerizable photoinitiators based on benzophenone. By this route, the photoinitiators are attached to the polymer, are not volatile, and can be effectively UV-crosslinked as a result of the binding to the polymer. A similar path was taken by Guse (U.S. Pat. No. 4,144,157). These acrylic PSAs are readily UV-crosslinkable and can be processed as hotmelts, but as a result of the broad molecular weight distribution they do not possess good technical adhesive properties.
A further variant is the RAFT process (Reversible Addition-Fragmentation Chain Transfer). The process is described at length in WO 98/01478 and WO 99/31144, but in the manner set out therein is unsuited to the preparation of PSAs, since the conversions achieved are very low and the average molecular weight of the polymers prepared is too low for acrylic PSAs. Accordingly, the polymers described cannot be used as acrylic PSAs. An improvement was achieved with the process described by BDF in DE 100 30 217.3.
The above-described process cannot, however, be used for UV crosslinking, since the compounds described likewise possess a radical scavenger effect, so that the crosslinking efficiency following addition of the free UV photoinitiator is too low. Moreover, there is a risk that the abovementioned types of compound will be unstable over a prolonged periodxe2x80x94such as is necessarily the case in a hotmelt process, for examplexe2x80x94and would decompose.
A central problem which therefore remains is the efficient UV crosslinking of narrow-distribution acrylic PSAs for the purpose of preparing improved acrylic PSAs.
It is an object of the invention, therefore, to provide a polyacrylate composition which has very good hotmelt processing properties and thereafter can be crosslinked very effectively, and also a process for preparing such UV-crosslinkable acrylic hotmelt pressure sensitive adhesives, which does not have the disadvantages of the aforementioned prior art, or at least not to so great an extent.
Surprisingly it has been found that narrow-distribution polyacrylate hotmelt pressure sensitive adhesives with copolymerized photoinitiators can be processed very effectively in a melt process and can be crosslinked very efficiently by UV crosslinking, and that they can be prepared outstandingly in a specially regulated process.
According to one aspect of the invention there is provided a polyacrylate pressure sensitive adhesive which has an average molecular weight Mw (weight average) of from 100 000 to 600 000 g/mol, possesses a polydispersity of not more than 3.0, and contains copolymerized photoinitiator units.
For the properties of the polyacrylate PSA it is very advantageous if one or more resins are admixed, preferably in fractions of up to 50% by weight, very preferably from 20 to 40% by weight.
It is further advantageous to add additives such as aging inhibitors, light stabilizers, ozone protectants, fatty acids, plasticizers, nucleators, blowing agents, accelerators, fillers and/or the like.
The invention further provides a process suitable for preparing polyacrylate pressure sensitive adhesives having a polydispersity of not more than 3.0. This is done by way of a radical polymerization process, in which a monomer mixture containing copolymerizable photoinitiators is polymerized in a process regulated by the presence of at least one chemical compound comprising the unit 
in which X is S, O or N.
Polymerization regulators of this kind which can be used very advantageously for the purposes of the invention include trithiocarbonates and dithioesters.
In a further development of the inventive process, following polymerization, which is preferably conducted up to a conversion of  greater than 98%, the polymer is concentrated to a hotmelt, the solvent being removed down to a maximum residual level of 0.1%, so that the polymer is in the form of a melt. In a further development of the process, the polymer is subsequently coated in gel-free form from the melt onto a backing (xe2x80x9cgel-freexe2x80x9d denotes compliance with the requirements for coatability of the compositions with the coating apparatus that is commonly used and is familiar to the skilled worker for these purposes, and particularly the requirement for coatability, featuring a uniform (homogeneous) coating pattern with no inhomogeneities or streaks when using the customary coating nozzles).
It is then advantageous to crosslink the polymer by UV radiation, particularly after coating onto the backing. In a preferred procedure here, the UV crosslinking is assisted by the added polymerization regulator.
In summary, the following scheme can be drawn up for an advantageous procedure:
polymerization of a monomer mixture containing not only monomers based on (meth)acrylic acid but also copolymerizable photoinitiators,
with polydispersities of from 1.2 to 3.5 being achieved through the use of a control reagent,
the polymer is concentrated to a hotmelt,
the polymer can be processed without gelling for 24 h in the hotmelt process in the absence of air or under nitrogen,
the polymer is coated gel-free from the melt, and
after coating, crosslinking is carried out with UV light, the added regulator assisting and accelerating UV crosslinking.
The UV-crosslinking, narrow-distribution polyacrylate PSA is preferably composed of the following monomers:
a) acrylates and/or methacrylates and/or their free acids with the following formula
CH2xe2x95x90CH(R1)(COOR2),
xe2x80x83where R1 is H or CH3 and R2 is an alkyl chain having 1-30 carbon atoms or H, at 70-99.9% by weight, especially 75-99.5% by weight,
b) UV photoinitiator containing a radically polymerizable double bond, at 0.1-2% by weight, especially 0.4-1% by weight,
c) olefinically unsaturated monomers containing functional groups, at 0-30% by weight.
In one very preferred version, the monomers a) used are acrylic monomers, comprising acrylic and methacrylic esters with alkyl groups composed of from 4 to 14 carbon atoms, preferably from 4 to 9 carbon atoms. Specific examples, without wishing to impose any unnecessary restriction by such a list, include n-butyl acrylate, n-pentyl acrylate, n-hexyl acrylate, n-heptyl acrylate, n-octyl acrylate, n-nonyl acrylate, lauryl acrylate, stearyl acrylate, behenyl acrylate, and the branched isomers thereof, such as 2-ethylhexyl acrylate, for example. Further classes of compound which may likewise be added in small amounts under a) are methyl methacrylates, cyclohexyl methacrylates, and isobornyl methacrylates.
In one very preferred version, photoinitiators used for the monomers b) are those comprising at least one vinyl compound. The photoinitiators may be of the Norrish I or Norrish II type. As building blocks the photoinitiators contain preferably one or more of the following radicals: benzophenone, acetophenone, benzyl, benzoin, hydroxyalkylphenone, phenyl cyclohexyl ketone, anthraquinone, trimethylbenzoylphosphine oxide, methylthiophenyl morpholine ketone, aminoketone, azobenzoin, thioxanthone, hexaarylbisimidazole, triazine, or fluorenone, it being possible additionally for each of these radicals to be substituted by one or more halogen atoms and/or one or more alkoxy groups, and/or one or more amino groups and/or hydroxyl groups. A representative overview is given in xe2x80x9cPhotoinitiation Photopolymerization and Photocuring, Fundamentals and Applicationsxe2x80x9d, by J. -P. Fouassier, Hanser Publishers, Munich, Vienna, N.Y. 1995. Further details can be gleaned from xe2x80x9cChemistry and Technology of UV and EB formulation for Coatings, Inks and Paintsxe2x80x9d, Volume 5, A. Carroy, C. Decker, J. P. Dowling, P. Pappas, B. Monroe, ed. by P. K. T. Oldring, publ. by SITA Technology, London, England 1994.
Specific examples, without wishing to be restricted unnecessarily as a result, include acrylated benzophenone, such as Ebecryl P 36(trademark) from UCB, or benzoin acrylate.
In one very peferred version the monomers c) used are vinyl esters, vinyl ethers, vinyl halides, vinylidene halides, vinyl compounds with aromatic cycles and heterocycles in the xcex1 position. Here again, a number of nonexclusive examples may be mentioned: vinyl acetate, vinylformamide, vinylpyridine, ethyl vinyl ether, vinyl chloride, vinylidene chloride and acrylonitrile. In a further very preferred version for the monomers c) monomers are used containing the following functional groups: hydroxyl, carboxyl, epoxy, acid amide, isocyanato or amino groups.
In one advantageous variant, acrylic monomers of the following general formula are used for c): 
where R1xe2x95x90H or CH3 and the radical xe2x80x94OR2 represents or comprises the functional group and, in one particularly preferred embodiment, possesses, for example, an H-donor effect which facilitates UV crosslinking.
Particularly preferred examples of component c) are hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, allyl alcohol, maleic anhydride, itaconic anhydride, itaconic acid, acrylamide and glyceridyl methacrylate, benzyl acrylate, benzyl methacrylate, phenyl acrylate, phenyl methacrylate, t-butylphenyl acrylate, t-butylphenyl methacrylate, phenoxyethyl acrylate, phenoxyethyl methacrylate, 2-butoxyethyl methacrylate, 2-butoxyethyl acrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, diethylaminoethyl methacrylate, diethylaminoethyl acrylate, cyanoethyl methacrylate, cyanoethyl acrylate, glyceryl methacrylate, 6-hydroxyhexyl methacrylate, N-tert-butylacrylamide, N-methylolmethacrylamide, N-(butoxymethyl)methacrylamide, N-methylolacrylamide, N-(ethoxymethyl)acrylamide, N-isopropylacrylamide, vinylacetic acid, tetrahydrofurfuryl acrylate, xcex2-acryloyloxypropionic acid, trichloroacrylic acid, fumaric acid, crotonic acid, aconitic acid, dimethylacrylic acid, this list not being conclusive.
In a further preferred version aromatic vinyl compounds are used for component c), the aromatic nuclei preferably being from C4 to C18 and further containing heteroatoms if desired. Particularly preferred examples are styrene, 4-vinylpyridine, N-vinylphthalimide, methylstyrene, 3,4-dimethoxystyrene, 4-vinylbenzoic acid, this list not being conclusive.
For polymerization the monomers are chosen such that the resulting polymers can be used as industrial PSAs, especially in such a way that the resulting polymers possess PSA properties in accordance with the xe2x80x9cHandbook of Pressure Sensitive Adhesive Technologyxe2x80x9d by Donatas Satas (van Nostrand, N.Y. 1989). For these applications the static glass transition temperature of the resulting polymer is advantageously below 25xc2x0 C.
For the polymerization it is preferred to use a control reagent of the general formula: 
in which
R and Rxe2x80x2 are chosen independently of one another or are identical and are
branched and unbranched C1 to C18 alkyl radicals; C3 to C18 alkenyl radicals; C3 to C18 alkynyl radicals;
H or C1 to C18 alkoxy;
C3 to C18 alkynyl radicals, C3 to C18 alkenyl radicals, C1 to C18 alkyl radicals substituted by at least one OH group or a halogen atom or a silyl ether;
C2-C18 heteroalkyl radicals having at least one oxygen atom and/or an NRxe2x80x2 group in the carbon chain;
C3 to C18 alkynyl radicals, C3 to C18 alkenyl radicals, C1 to C18 alkyl radicals substituted by at least one ester group, amine group, carbonate group, cyano group, isocyanato group and/or epoxide group and/or by sulfur;
C3-C12 cycloalkyl radicals;
C6-C18 aryl or benzyl radicals;
hydrogen.
In one more preferred version, control reagents of type (I) consist of the following compounds:
halogens are preferably F, Cl, Br or I, more preferably Cl and Br. As alkyl, alkenyl and alkynyl radicals in the various substituents, both linear and branched chains are outstandingly suitable.
Examples of alkyl radicals containing from 1 to 18 carbon atoms are methyl, ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl, pentyl, 2-pentyl, hexyl, heptyl, octyl, 2-ethylhexyl, t-octyl, nonyl, decyl, undecyl, tridecyl, tetradecyl, hexadecyl, and octadecyl.
Examples of alkenyl radicals containing from 3 to 18 carbon atoms are propenyl, 2-butenyl, 3-butenyl, isobutenyl, n-2,4-pentadienyl, 3-methyl-2-butenyl, n-2-octenyl, n-2-dodecenyl, isododecenyl, and oleyl.
Examples of alkynyl containing from 3 to 18 carbon atoms are propynyl, 2-butynyl, 3-butynyl, n-2-octynyl, and n-2-octadecynyl.
Examples of hydroxy-substituted alkyl radicals are hydroxypropyl, hydroxybutyl, and hydroxyhexyl.
Examples of halogen-substituted alkyl radicals are dichlorobutyl, monobromobutyl, and trichlorohexyl.
An example of a suitable C2-C18 heteroalkyl radical containing at least one oxygen atom in the carbon chain is xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94CH3.
Examples of C3-C12 cycloalkyl radicals include cyclopropyl, cyclopentyl, cyclohexyl, and trimethylcyclohexyl.
Examples of C6-C18 aryl radicals include phenyl, naphthyl, benzyl, 4-tert-butylbenzyl or further substituted phenyl, such as ethylbenzene, toluene, xylene, mesitylene, isopropylbenzene, dichlorobenzene or bromotoluene.
The above listings serve only as examples of the respective groups of compounds, and make no claim to completeness.
Further suitable control reagents include compounds of the following types: 
in which Rxe2x80x3 may comprise the abovementioned radicals R or Rxe2x80x2, independently of their selection.
In one particularly preferred embodiment of the invention compounds (Ia) and (IIa) are used as control reagents. 
In connection with the abovementioned polymerizations proceeding by a controlled radical process, preference is given to using initiator systems which further comprise further radical initiators for the polymerization, especially thermally decomposing, radical-forming azo or peroxo initiators. In principle, all customary initiators which are known for acrylates are suitable for this purpose. The production of C-centered radicals is described in Houben Weyl, Methoden der Organischen Chemie, Vol. E 19a, pp. 60-147. These methods are employed, preferentially, in analogy.
Examples of radical sources are peroxides, hydroperoxides, and azo compounds; some nonlimiting examples of typical radical initiators that may be mentioned here include potassium peroxodisulfate, dibenzoyl peroxide, cumene hydroperoxide, cyclohexanone peroxide, di-t-butyl peroxide, azodiisobutyronitrile, cyclohexylsulfonyl acetyl peroxide, diisopropyl percarbonate, t-butyl peroctoate, and benzpinacol. In one very preferred version, 1,1xe2x80x2-azobis(cyclohexanecarbonitrile) (Vazo 88(trademark) from DuPont) is used as radical initiator.
The average molecular weights Mw (weight average) of the polymers formed in the controlled radical polymerization are chosen so that they are situated within a range from 100 000 to 600 000 g/mol; specifically for further use as hotmelt PSAs, adhesives having average molecular weights (weight average) Mw of from 100 000 to 350 000 g/mol are prepared. The average molecular weight Mw is determined by size exclusion chromatography (gel permeation chromatography, GPC) or matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS).
The polymerization may be conducted in bulk, in the presence of an organic solvent or mixtures of organic solvents, in the presence of water, or in mixtures of organic solvents and water. The aim is to minimize the amount of solvent used. Suitable organic solvents or solvent mixtures are pure alkanes (hexane, heptane, octane, isooctane), aromatic hydrocarbons (benzene, toluene, xylene), esters (ethyl, propyl, butyl or hexyl acetate), halogenated hydrocarbons (chlorobenzene), alkanols (methanol, ethanol, ethylene glycol, ethylene glycol monomethyl ether), ethers (diethyl ether, dibutyl ether) or mixtures thereof. A water-miscible or hydrophilic cosolvent may be added to the aqueous polymerization reactions in order to ensure that the reaction mixture is present in the form of a homogeneous phase during monomer conversion. Cosolvents which can be used with advantage for the present invention are chosen from the following group, consisting of aliphatic alcohols, glycols, ethers, glycol ethers, pyrrolidines, N-alkylpyrrolidinones, N-alkylpyrrolidones, polyethylene glycols, polypropylene glycols, amides, carboxylic acids and salts thereof, esters, organic sulfides, sulfoxides, sulfones, alcohol derivatives, hydroxy ether derivatives, amino alcohols, ketones and the like, and also derivatives and mixtures thereof.
The polymerization timexe2x80x94depending on conversion and temperaturexe2x80x94is between 4 and 72 hours. The higher the reaction temperature which can be chosen, i.e., the higher the thermal stability of the reaction mixture, the shorter the possible reaction time.
As regards initiation of the polymerization, the introduction of heat is essential for the thermally decomposing initiators. For these initiators the polymerization can be initiated by heating to from 50 to 160xc2x0 C., depending on initiator type.
For the use of the polymers prepared by the inventive process (polyacrylates) as pressure sensitive adhesives, they are optimized by optional blending with at least one resin. Tackifying resins to be added include without exception all existing tackifier resins described in the literature. Representatives that may be mentioned include pinene resins, indene resins, and rosins, their disproportionated, hydrogenated, polymerized, esterified derivatives and salts, the aliphatic and aromatic hydrocarbon resins, terpene resins and terpene-phenolic resins, and also C5, C9 and other hydrocarbon resins. Any desired combinations of these and other resins may be used in order to adjust the properties of the resulting adhesive in accordance with what is desired. In general it is possible to use all resins which are compatible (soluble) with the corresponding polyacrylate; mention may be made in particular of all aliphatic, aromatic, alkylaromatic hydrocarbon resins, hydrocarbon resins based on pure monomers, hydrogenated hydrocarbon resins, functional hydrocarbon resins, and natural resins. Explicit reference is made to the depiction of the state of the art in the xe2x80x9cHandbook of Pressure Sensitive Adhesive Technologyxe2x80x9d by Donatas Satas (van Nostrand, 1989).
In a further advantageous development one or more plasticizers are added to the polyacrylates, such as low molecular mass polyacrylates, phthalates, whale oil plasticizers or plasticizer resins, for example.
The polyacrylates may further be blended with one or more additives such as aging inhibitors, light stabilizers, ozone protectents, fatty acids, resins, nucleators, blowing agents, compounding agents and/or accelerators. The aging inhibitors are, in particular, primary and secondary aging inhibitors, which are available commercially under the trade names Irganox(trademark) from Ciba Geigy and Hostanox(trademark) from Clariant.
They may further be admixed with one or more fillers such as fibers, carbon black, zinc oxide, titanium dioxide, solid or hollow glass (micro)beads, microbeads of other materials, silica, silicates, and chalk, with the addition of blocking-free isocyanates being a further possibility.
Particularly for use as a pressure sensitive adhesive it is of advantage for the inventive process if the polyacrylate (the resulting polymer) is applied preferably from the melt, gel-free, to a backing or to a backing material, as a film.
For this purpose the polyacrylates prepared as described above are concentrated to give a polyacrylate composition whose solvent content is xe2x89xa62% by weight, with particular preference xe2x89xa60.5% by weight. This process takes place preferably in a concentrating extruder. Then, in one advantageous variant of the process, the polyacrylate composition is applied in the form of a film, as a hotmelt composition, to a backing or to a backing material.
Backing materials used for the PSA, for adhesive tapes for example, are the materials customary and familiar to the skilled worker, such as films (polyesters, PET, PE, PP, BOPP, PVC), nonwovens, foams, wovens and woven films, and also release paper (glassine, HDPE, LDPE). This list is not conclusive.
For the PSA utility it is particularly advantageous to crosslink the polyacrylates following application to the backing or to the backing material. For this purpose, in order to produce the PSA tapes, the polymers described above are optionally blended with crosslinkers. Preferred radiation-crosslinking substances in accordance with the process of the invention are, for example, difunctional or polyfunctional acrylates or difunctional or polyfunctional urethane acrylates, difunctional or polyfunctional isocyanates or difunctional or polyfunctional epoxides. However, it is also possible here to use any other difunctional or polyfunctional compounds which are familiar to the skilled worker and are capable of crosslinking polyacrylates.
In order to improve the crosslinking efficiency, the polyacrylates may where appropriate be blended with further, uncopolymerized photoinitiators. Those suitable for this purpose preferably include Norrish Type I and Type II cleavers, some possible examples of both classes being benzophenone, acetophenone, benzyl, benzoin, hydroxyalkylphenone, phenyl cyclohexyl ketone, anthraquinone, thioxanthone, triazine, or fluorenone derivatives, this list making no claim to completeness. A representative overview is again given in xe2x80x9cPhotoinitiation Photopolymerization and Photocuring, Fundamentals and Applicationsxe2x80x9d, by J. -P. Fouassier, Hanser Publishers, Munich, Vienna, N.Y. 1995. For further details consult xe2x80x9cChemistry and Technology of UV and EB formulation for Coatings, Inks and Paintsxe2x80x9d, Volume 5, A. Carroy, C. Decker, J. P. Dowling, P. Pappas, B. Monroe, ed. by P. K. T. Oldring, publ. by SITA Technology, London, England 1994.
UV crosslinking takes place very preferably by means of brief ultraviolet irradiation in a wavelength range from 200 to 450 nm, especially using high or medium pressure mercury lamps with an output of from 80 to 240 W/cm. For UV crosslinking it is, however, also possible to use monochromatic radiation in the form of lasers. In order to prevent instances of overheating it may be appropriate to shade off part of the UV beam path. It is also possible to use special reflector systems which act as cold light emitters in order thereby to prevent overheating.
It may be appropriate to crosslink the polyacrylates described in accordance with the invention using electron beams as well. Typical irradiation equipment which may be used includes linear cathode systems, scanner systems or segmented cathode systems, where electron beam accelerators are concerned. A detailed description of the state of the art and of the major process parameters can be found in Skelhorne, xe2x80x9cElectron Beam Processingxe2x80x9d in Vol. 1 xe2x80x9cChemistry and Technology of UV and EB Formulations for Coatings, Inks and Paintsxe2x80x9d published by Sita Technology, London 1991. The typical accelerating voltages are situated in a range between 50 kV and 500 kV, preferably from 80 kV to 300 kV. The radiation doses employed range between 5 to 50 kGy, in particular from 20 to 100 kGy.
The invention further provides for the use of the polyacrylate pressure sensitive adhesive for an adhesive tape, in which case the polyacrylate pressure sensitive adhesive may have been applied to one or both sides of a backing.